Motor vehicle hydraulic brake systems include a master cylinder having a piston which reciprocates in a cylinder bore to pressurize a brake fluid. A brake fluid reservoir communicates with the cylinder bore to provide additional brake fluid as required. The typical brake fluid reservoir has a fill line or indicator to designate the level the brake fluid shall not be filled beyond.
The master cylinder piston in such brake systems typically carries an elastomeric seal which seals the piston to the cylinder bore. The entry of a foreign fluid into the brake fluid may cause the piston seal to swell. Likewise, the wheel brakes have wheel cylinders with pistons which carry elastomeric seals. The wheel brake pistons seals may also swell in the presence of a foreign fluid.
It would be desirable to provide an indicator of the presence of such a foreign fluid in the brake fluid.